My thoughts are in blue, words to delete are in red, suggested rephrasing is in orange.

When her sister becomes possessed by the ghost of a powerful witch, Luna is forced to become the strong twin and find a way to save Aurora before she is consumed completely (to eliminate "witch" echo.) the witch consumes her completely. Overall, good hook!

Thirteen-year-old twins Luna and Aurora are as different as night and day, but have always shared a special bond. Now Luna fears they are growing apart, with different interests and friendship groups. When they travel with their mother to remote Orkney to bury their grandmother and clear out her little cottage overlooking Eynhallow Sound, Luna is sad, but relieved to have her sister all to herself again. Good setup, but clarify - why is Luna sad? The death of her grandmother? The growing divide between the sisters? Both? Clarity would be good. Also, you infer that Luna is the "weaker" of the two, so maybe illustrate that here. Do you mean she's less popular? Not as assertive? How is she the "weaker" of the two?

However, Luna soon becomes aware of something that threatens to take her sister away from her forever. Aurora, who has never feared anything, is haunted by owls and tormented by frightening dreams. She seems to have acquired some magical power, able to weave her fingers and call up a violent storm. Aurora is terrified and extremely unwell How? Mentally? Physically? Emotionally? and Luna realises their roles have become reversed, and she must be the one to comfort and protect her twin. She discovers their grandmother accidently disturbed an ancient stone, releasing the ghost of a powerful weather witch who is determined to take over her sister’s life-force so she can come back into the world.

Every day the witch grows stronger and Rory I would just stick to her full name. I had to wonder who Rory was for a second fades. With the guidance of the last few witches left in Orkney, who use their powers to protect and nurture the land and its heritage, Luna searches for the identity of the ancient witch, and the secrets of her binding, so she can try to replicate it. I don't know what this means, or why she would want to replicate something that sounds bad / dangerous But time is running out, the witch is wary and will not fall for the same trick twice. This infers that she's fallen for something before. From someone else? From Luna? She lashes out with power, anger and a thirst for revenge, creating ferocious storms and floods which injure the girls’ mother and put the whole community in danger. When every other possibility has been exhausted, Luna must find the courage to look in the mirror which was the object of binding, and connect with the witch’s memories, even though she may not be able to pull free again and the witch might come for her as well.

THE WITCH OF EYNHALLOW SOUND is a Middle Grade Paranormal Fantasy set in Orkney, complete at 30,000 words, which should appeal to readers who enjoyed the sinister darkness of The Night Gardener, the ghostly presences in The Forgotten Girl and the lyrical magical setting of The Storm Keeper’s Island.

I have published a number of literary travel articles, mainly in Good Reading Magazine (Australia), about visiting the places that inspired my favourite authors.

Good comp titles and bio. I feel like your word count might be a touch light for a fantastical MG that will need worldbuilding. Reference: http://literaticat.blogspot.com/2011/05/wordcount-dracula.html I don't know that it will kill you in the query process, but it made me question whether the story, setting, and characters are fully fleshed out.